Integrated circuits (ICs) are often very complex devices from both a structural and functional standpoint. The testing of such structurally and functionally complex devices is often equally complex. In order to ensure the IC is functioning properly, manufacturers test the devices on sophisticated testing machines. Some ICs are placed into a test mode of operation in order to be properly and expediently tested. These ICs are operated in their final or end use in a normal or non-test mode of operation. Thus, the IC must be placed into a desired one of at least two different operational modes.
Typically, an IC is placed into a desired mode of operation through the application of a predetermined sequence of several mode selection signals to the input pins of the IC. In order for mode selection or other input signals to be received and decoded by the IC, the IC must be powered up and ready to accept input signals. This requires the IC to complete its power on or start-up process, which typically includes turning on voltage pumps, initializing chip logic, etc. The predetermined sequence of mode selection signals sent to the IC to invoke the desired mode of operation generally includes signals that set the operational mode register of the IC and thereby cause the IC to enter a desired “generic” operational mode. Further mode selection signals, referred to as mode entry keys, are provided to select a specific test or operational mode within the generic mode of operation.
The first mode selection signal is compared against a predetermined code. If the mode selection signal matches the predetermined code, the next signal is received as a mode entry key and decoded as such to thereby place the IC into a specific mode of operation that corresponds to the particular mode entry key. The mode selection signal, in essence, verifies that the signal to follow will be a mode entry key and will therefore correspond to a specific mode of operation. Thus, in order to place a conventional IC into a specific mode of operation, at least two separate mode selection signals, i.e., a mode selection signal and a mode entry key, must be issued and decoded.
The above-described signals are typically applied by a piece of equipment, such as, for example, a test machine or test station, which must be configured to apply the appropriate signals in the proper sequence that are required to place the particular IC into the desired operational mode. If, for example, the manufacturer desires to test a different IC on the same piece of test equipment that piece of test equipment must be reconfigured to apply the specific signals in the proper sequence to place that particular IC in the desired mode of operation. The required set up of the machine and the application of the signals to the IC consumes valuable time.
The first time, or first few times, an IC is powered up and operated is likely to be for the purpose of testing. However, each IC must nonetheless be individually placed into the desired mode of operation through the time-consuming and laborious process described above. Once the initial testing of the IC has been completed, subsequent entry into the test mode of operation is typically not necessary and may be undesirable.
Therefore, what is needed in the art is a method and apparatus that places an IC into a desired mode of operation in a reduced amount of time.
Furthermore, what is needed in the art is a method and apparatus that places an IC into a desired mode of operation without requiring the application of a plurality of input signals to the IC.
Moreover, what is needed in the art is a method and apparatus that places an IC into a default mode of operation and which can be reset to automatically enter a non-default mode of operation to thereby prevent inadvertent entry of the IC into the default mode of operation.